Field of the Invention
The present invention relates to an electroconductive member, a process cartridge, and an electrophotographic image-forming apparatus.
Description of the Related Art
In an electrophotographic image-forming apparatus, electroconductive members for electrophotography, such as a charging member, a developing member, and a transferring member, are used.
In the case of a charging roller, the charging member is required to have an ability to uniformly charge a body to be charged, such as a photosensitive member.
In recent years, there have been demands for an additional improvement in image quality of an electrophotographic image, and a higher process speed and higher durability of the electrophotographic image-forming apparatus. Consequently, charging unevenness of the photosensitive member even at a level at which no problem has occurred heretofore may cause an image defect.
In particular, in recent years, a proposal of adopting a cleaner-less system (toner recycling system) in the electrophotographic image-forming apparatus has been made from the viewpoints of simplifying an image-forming apparatus and eliminating waste.
In this system, a drum cleaner serving as a cleaning unit after a transferring step is eliminated, and transfer residual toner on the photosensitive member after transfer is cleaned out by a developing apparatus simultaneously with development, to thereby remove the transfer residual toner from the photosensitive member and cause the developing apparatus to recover the transfer residual toner.
The method involving cleaning out the transfer residual toner on the photosensitive member simultaneously with development is a method involving recovering the transfer residual toner on the photosensitive member during development of an electrostatic latent image on the photosensitive member before proceeding to the next step of forming an electrophotographic image, through the use of a fog-removing bias, that is, a fog-removing voltage difference (Vback) which is a potential difference between a DC voltage to be applied to the developing apparatus and a surface potential of an image-bearing member.
In this connection, when a charging member of a contact charging system is applied to the cleaner-less system, adhesion of contamination to a surface of the charging member becomes more remarkable in some cases. In order to solve such problem, there is a proposal of a system in which a circumferential speed difference is provided between the charging member and the photosensitive member and the transfer residual toner is triboelectrically charged by the charging member, with the result that it is electrostatically difficult for the transfer residual toner to migrate to the surface of the charging member. However, the provision of the circumferential speed difference between the charging member and the photosensitive member may increase injection charging of the photosensitive member by the charging member, with the result that charging unevenness may be liable to occur on the photosensitive member.
In this regard, in order to reduce the injection charging of the photosensitive member by the charging member, in Japanese Patent Application Laid-Open No. 2010-72405, there is a proposal of adding a hydrophobic surfactant into a surface layer of a charging roller.
In addition, in Japanese Patent Application Laid-Open No. H05-323762, there is a proposal of a charging sheet formed of two layers, specifically, an insulating layer and an electroconductive layer.
According to investigations made by the inventors of the present invention, when the hydrophobic surfactant is added to the surface layer of the charging roller as in Japanese Patent Application Laid-Open No. 2010-72405, an effect is obtained on the injection charging of the photosensitive member at an initial stage of image output, but the injection charging-suppressing effect disappears along with an increase in number of sheets subjected to image output. The reason for this is considered to be as described below. The hydrophobic surfactant is oriented to the outermost surface of the surface layer of the charging roller to suppress the injection charging. However, along with the increase in number of sheets subjected to image output, the hydrophobic surfactant at the outermost surface of the surface layer of the charging roller may migrate to the photosensitive member in contact therewith, or the hydrophobic surfactant may be decomposed by a discharge, resulting in the disappearance of the injection charging-suppressing effect. In addition, owing to a small amount of the hydrophobic surfactant added to the surface layer of the charging roller, a binder of the surface layer significantly affects the injection charging of the photosensitive member, and hence binders which may be used for the surface layer are limited.
In addition, according to investigations made by the inventors of the present invention, when the charging sheet of Japanese Patent Application Laid-Open No. H05-323762 is used, the injection charging of the photosensitive member can be suppressed. However, a fine gap region in which a discharge occurs, the region being formed by the charging sheet and the photosensitive member brought into contact with each other, is liable to change owing to film thickness unevenness of the photosensitive member or powder of, for example, an external additive or toner remaining on the photosensitive member. Accordingly, when the fine gap region changes, the discharge region changes, resulting in change in ease of potential transfer. As a result, the photosensitive member is liable to have potential unevenness owing to discharge unevenness.
In view of the foregoing, the present invention is directed to providing an electroconductive member for electrophotography capable of more uniformly charging a body to be charged. The present invention is also directed to providing a process cartridge and an electrophotographic image-forming apparatus which are capable of stably forming high-quality electrophotographic images over a long period of time.